I remember watching my uncle playing with a home-built oscilloscope in the early 1950s. At the time, I was intrigued by the green glow of the P7 phosphor making Lissajous figures in the darkened room. Later, I rebuilt that same instrument -- modeled on the Dumont 304 -- on its World War II surplus radar chassis as a high school science project. Reflecting on this, I realize that I've been witness to the full development of oscilloscope measurement technology.
Measurements on the early oscilloscopes, such as the Dumont 304, were all based on a rectangular grid laid over the face of the CRT (cathode ray tube). Reading a signal's amplitude involved counting the number of grid divisions between the maximum and minimum excursions of the trace and multiplying by the scopeís vertical scale setting (usually expressed in Volts/division). Similarly, the period of a waveform was measured by counting the number of horizontal division between edges of the signal and multiplying by the sweep speed (in seconds/division). This was complicated by the fact that the grid was usually etched on a plastic sheet that was not coplanar with the CRT screen. Moving your head resulted in parallax error.
Oscilloscope manufacturers fixed that by etching the grid on the glass screen of the CRT. They also added some helpful lines on the grid to help mark the top and base of pulses and help users estimate the 10% and 90% levels for measuring pulse rise time. That classic grid is still with us today as shown in the figure.
This screen image incorporates all the evolutionary elements of oscilloscope measurements, including the grid, cursors, and measurement parameters with statistics.
(Click here to see a larger image.)
The grid in the figure consists of eight vertical divisions (some oscilloscopes use ten) and ten horizontal divisions. The centerlines of both axes have five tick marks per division to provide intermediate scaling. Note the dotted lines one and a half divisions below the top and above the bottom edges of the grid. These lines are exactly five divisions apart. If you adjust the height of the waveform to fill the area between the dotted lines, then the solid grid lines just inside of the dotted lines mark the 10% and 90% amplitude levels. Measuring the time between the points where the waveform crosses those solid lines gives you the rise time of the waveform. Try estimating the rise time using only the grid.
In the early 1970s, cursors began to appear on analog oscilloscopes. The first cursors were horizontal and vertical lines superimposed on the display that you could use to measure the amplitude and time differences of points on the waveform. Cursors couldn't do more on an analog oscilloscope since the waveforms were transient -- they only existed for the duration of the sweep. The advent of the digital oscilloscope in the late 1970s changed that. Waveforms were digitized and stored. Cursors like the ones shown above tracked the waveform (note the up and down arrow icons) and precise readouts of the absolute and differential amplitudes and times are shown.
The stored waveforms in digital oscilloscopes also allowed the computation of signal measurement parameters which entered the picture in the 1980s. Based on standards such as IEEE 181, statistical analysis of the waveforms provided accurate readouts of pulse amplitudes and timing. Additionally, storage of multiple measurement results allowed for the calculation and display of statistics. Parameter P1 in the figure displays the signal rise time along with the mean, maximum, minimum, and standard deviation of over 10,000 measurements. The histogram of the rise-time measurement is also shown in the iconic "histicon" beneath the parameter readouts.
The statistical basis for the measurement parameters makes them the most accurate tools for waveform measurements. Cursors and even "box" counting still serve in many instances where specific parameters donít exist or where your measurements are based on a printed image of a long ago signal acquisition with no parameters recorded. The next time you use your oscilloscope, appreciate the work of hundreds of engineers who brought you such a useful tool.